1. Technical Field
The present invention relates to a silicone rubber composition which evidences an excellent moldability, which can be molded without the use of special molds into silicone rubber moldings whose dimensions are the same as for the use of a conventional silicone rubber composition, which supports the facile removal of low-molecular-weight organosiloxane after molding, and which provides silicone rubber moldings which are free of the problems associated with low-molecular-weight organosiloxane, for example, defective conduction at electrical switch contacts and so forth. The present invention also relates to a method for the production of the aforesaid silicone rubber moldings.
2. Prior Art
Diorganopolysiloxane-based silicone rubber compositions offer a good heat resistance, cold resistance, and chemical resistance, as well as, an excellent electrical insulation, and as a consequence are used in a wide variety of electrical equipment and devices as a heat-resistant insulating material. Moreover, these compositions are used in electrical equipment as conductive materials through the addition to them of electrically conductive filler. However, the moldings obtained from such silicone rubber compositions exercise a negative influence on electrical switch contacts which may be present nearby and in fact often cause conduction faults at electrical contacts. The following mechanism has been reported for this phenomenon: residual low-molecular-weight organosiloxane in the silicone rubber molding evaporates, either at room temperature or upon heating; this gas, upon reaching an electrical switch contact, is exposed to the discharge energy associated with operation of the contact; and chemical transformations are induced which result in the formation of insulating substances such as silicon dioxide and silicon carbide, see Denki Tsushin Gakkai Gijutsu Kenkyu Hokoku, 76 (226) pp. 29-38 (1977).
In one method for suppressing the defective conduction at electrical switch contacts which is caused by this low-molecular-weight organosiloxane gas, the molding prepared by the thermal cure (primary vulcanization) of an ordinary silicone rubber composition is subjected to a secondary vulcanization for completion of the cure at a temperature higher than the 150.degree. C. to 220.degree. C. used in ordinary secondary vulcanization for a period of time longer than the 1 to 12 hours in ordinary secondary vulcanization in order to remove the low-molecular-weight siloxane present in the molding. In addition, investigations have been conducted into methods for obtaining silicone rubber moldings which do not cause conduction faults at electrical contacts through the use of silicone rubber compositions based on diorganopolysiloxane which has been depleted of low-molecular-weight organosiloxane.
However, in the former approach, since it is difficult to accomplish the satisfactory removal of low-molecular-weight organosiloxane from silicone rubber moldings using ordinary or conventional secondary vulcanization conditions, heat treatment at high temperatures (220.degree. C. to 270.degree. C.) for long periods of time (at least 24 hours) is required in order to remove low-molecular-weight organosiloxane to a degree which eliminates defective electrical contact conduction. This accrues the disadvantages of a reduction in productivity and an increase in product cost.
In the latter approach, when the silicone rubber compositions which are employed by this method are processed in those molds used heretofore for silicone rubber molding, moldings are obtained which have dimensions larger than those of the target molding. As a consequence, there have been attempts at the production of moldings with the same dimensions as before through the design and production of new, dedicated molds which are suitable only for such silicone rubber compositions. However, this approach entails cost disadvantages due to its requirements for new equipment investment.